الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 226 |  |  |
| | | from | 226 | | |
Abstract
The glasses defined by the formulae (13.5%X– 32.5 %V2O5–54%P2O5)
mol%, X = Na2O, Na2S or NaF, and (13.5% Na2S – 32.5 %V2S5–54%P2S5)
mol% were easily obtained by employing the melt-quenching technique. XRD approved the glassy nature of these samples. The as-synthesized (13.5% Na2S
– 32.5 %V2O5–54%P2O5) glass showed the highest electrical conductivity and was thermally treated at a precise temperature (530 °C) for different times (4, 6, and 12 h) to produce the desired glass-ceramic nanocomposites. The glass sample that is thermally treated for 4 hours (4h) was identified by XRD to be nano-crystalline rhombohedral Na3V2(PO4)3 NASICON-
structured embedded in the glass matrix. The 12h heat treated sample at the same temperature was identified by XRD to be nano-crystalline orthorhombic β-VOPO4. The crystallite size of the three nanocomposite samples is between 42–66 nm, where D decreased with increasing heat treatment time at the same temperature. The dc conductivity of the sulfur-containing glass was higher compared to the sulfur-free glasses where the replacement of oxygen with nonbridging sulfur atoms causes a slight reduction in the glass network connectivity that is favorable for ion movement. The dc conductivity also increased after S addition due to the increase of V4+–V5+ or V3+–V4+ ion pairs where S was employed as a reducing agent. The dc conductivity calculated for the glass-ceramic nanocomposites was found to have much higher values than those of the initial glass, this enhancement of conductivity values is ascribed to the creation of defective regions at the interfaces of crystallite/glass that promotes the electrical
conductivity. The calculated dc conductivity values for all glass-ceramic nanocomposites exhibited higher magnitudes with the decrease in crystallite size while the activation energy showed opposite behavior, this is ascribed to the decrease of charge carrier scattering at grain boundary. The dynamics of Na+ ions at a temperature window of 333 to 373 K in the as-prepared glasses were investigated at frequencies from 20 Hz to 1 MHz. The frequency exponent (s) is inversely proportional with temperature, this is consistent with Elliott’s correlated barrier hopping (CBH) model. The electrochemical tests of the prepared (13.5% Na2S – 32.5 %V2O5–
54%P2O5) mol% samples and its glass-ceramic nanocomposites as a cathode material for rechargeable ion battery was carried out at both room temperature and 55 °C. The β -VOPO4 sample showed favorable electrochemical performance after 100 cycles with a coulombic efficiency of about 78, 41℅ at room temperature and 55 °C, respectively with an energy density of 111 Wh kg-1 at 55 °C. Other samples showed poor electrochemical performances at both temperatures. The good electrochemical performance of the β -VOPO4
electrode can be related to the crystal structure of the main phase. The β-
VOPO4 possess a structure similar to VO5F octahedra and PO4 tetrahedra found in Na3V2O2F(PO4)2, this flexible structure provides a three-dimensional skeleton and thus constructing facile channels for the fast ion diffusion. The 12h electrode electrochemical behavior can also be ascribed to the nanocrystalline size (42 nm), the 12h sample has the smallest crystalline size and the most porous structure, and this can promote ions intercalation/deintercalation during cycling. Finally, the findings imply that the β -VOPO4 electrode can be a possible candidate for rechargeable ion battery industry.